German Patent Specification Nos. 878,544 and 889,349 relate to the coating of split leather or the priming of already tanned leather with solutions of a mixture of alkyd resins which contain free hydroxyl groups and isocyanates as cross-linking agents. These patent specifications, however, give no indication as to any tanning effect of reaction products of polyisocyanates and an excess of hydroxyl compounds.
German Patent Specification Nos. 853,438 and 857,425 describe, among other things, the tanning of leather with dispersions, solutions of vapors of low molecular weight polyisocyanates.
These processes generally produce soft leathers which are in many cases, also lightfast, buth they are unsuitable in practice because of the toxic properties of low molecular weight diisocyanates.
It has now surprisingly been found that high quality products may be produced without the disadvantage of the previously known tanning processes by treating animal pelts or leather which has been retanned in the conventional manner with an aqueous liquor containing water-soluble oligourethanes which contain ionic groups.